1. Field of the Invention
The present invention relates to a voltage generating circuit which is adapted for use in a semiconductor device having an on-chip power supply circuit and produces an internal supply voltage from an external supply voltage.
2. Description of the Related Art
In recent years, with advances in semiconductor manufacturing technology, the operating voltage of transistors has become increasingly low. Accordingly, it has become necessary to suppress variations in supply voltage within the chip. Up to now, a method has been adopted which involves connecting a capacitor having a large capacitance to an interconnect line which connects a power supply pad supplied with an external supply voltage with macro cells in order to suppress variations in supply voltage within the chip. However, the interconnect line between the power supply pad and the macro cells has an impedance, which may result in failure to suppress variations in supply voltage sufficiently.
For this reason, in recent years, a method has been adopted in which a voltage generating circuit, such as a DC-to-DC converter, is provided on a chip to produce an internal supply voltage from an external supply voltage. As the voltage generating circuit, use has been made of a dropper type regulator circuit or a switched-capacitor-based voltage dropping (stepdown) circuit in producing a voltage lower than an external supply voltage or a pump circuit or the like in producing a voltage higher than the external supply voltage. When a necessary internal supply voltage is comparable to an external supply voltage, both a stepdown circuit and a stepup circuit are provided on a chip. When the external supply voltage is higher than the internal voltage, the stepdown circuit is used to make the internal voltage lower the external supply voltage; otherwise, the stepup circuit is used to step up the external supply voltage. However, the provision of both the stepup circuit and the stepdown circuit results in an increase in the chip area.
Accordingly, a switched-capacitor type of voltage generating circuit has been developed which has a stepdown circuit and a stepup circuit. This voltage generating circuit is composed of two or more charge-transfer N-channel MOS transistors (hereinafter referred to as NMOS transistors) series connected between an input terminal supplied with an external supply voltage and an output terminal, capacitors each of which is connected between the node between the adjacent NMOS transistors and ground, and a capacitor connected between the output terminal and the series combination of the NMOS transistors. The voltage generating circuit produces a desired internal supply voltage by turning on and off the NMOS transistors in sequence starting with the transistor on the input side and thereby charging the capacitors in sequence.
The back gate of each NMOS transistor in the circuit is connected to ground (GND). In such a situation, the on resistance of the MOS transistor increases. For this reason, the current supply capability of the transistor falls. In order to reduce the on resistance of the transistor, one might suggest setting the back-gate bias a little higher.
In a voltage generating circuit arranged, for example, to produce an internal supply voltage (VINT) of 3.3 V from an external supply voltage (VEXT) of 3.3 V, when the tolerance for variations in the external supply voltage is 10%, it may fluctuate between 2.97 and 3.63 V. It is therefore required for the voltage generating circuit to perform both the stepdown and the stepup operation as the external supply voltage fluctuates. To this end, various methods are considered. For example, when the back-gate bias of an NMOS transistor is increased to lower its on resistance, a forward voltage is applied across its PN junction, which may cause its associated parasitic bipolar transistor to turn on and consequently a leakage current to flow. When the back-gate bias of an NMOS transistor is too low, its on resistance increases, resulting in reduced current supply capability. Thus, when the back-gate bias is made either high or low, the performance is degraded. Therefore, the demand is increasing for a voltage generating circuit which is capable of preventing the circuit performance from falling whether the internal supply voltage is higher or lower than the internal supply voltage.
A voltage generating circuit using switched capacitors is described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 07-212215. In addition, a back-gate bias producing circuit is described in, for example, U.S. Pat. No. 5,900,665, which is adapted to produce a back-gate bias according to an operating cycle of a semiconductor integrated circuit.